Costilla County, CO: EPA's radon zone, explained
EPA places Costilla County in Zone 2 - Moderate Potential — a predicted county-wide average indoor radon level between 2 and 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Nationally, Costilla County ranks higher in radon potential than 35% of rated counties (the 35th percentile).
Costilla County by the numbers
Costilla County, Colorado is home to about 3,686 people across 1,227.593 square miles. Population density there runs about 3 people per square mile. By population, Costilla County is the 972nd largest of every county sharing its EPA zone — the 6th percentile by population within that zone. EPA's geology-based Region 8 groups Costilla County with 290 other counties nationally sharing its underlying rock and soil profile.
Why the zone isn't the answer by itself
Costilla County's predicted average falls between EPA's 4 pCi/L action level and the national indoor average of about 1.3 pCi/L (outdoor air runs roughly 0.4 pCi/L). Costilla County's Zone 2 rating sits in the middle nationally; EPA's guidance is unchanged by that — test every home regardless of its zone.
Next steps for a home in Costilla County
For a Costilla County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.
Find Colorado's state radon program
For Costilla County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Costilla County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.
Where Costilla County's figures come from
Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. Whatever zone Costilla County carries, the zone explainer covers what it does and doesn't tell you about any one house.